Phytophthora ramorum Disease
Transmission From Artificially Infested
Potting Media1
Jennifer L. Parke, Melody L. Roth, Carrie Lewis, and
Caroline J. Choquette2
Abstract
Potted rhododendrons grown in potting media amended with inoculum of Phytophthora
ramorum became infected and showed symptoms of stem necrosis, leaf wilting, and death.
P. ramorum was isolated from roots and stems of infected plants.
Key words: nursery, potting media, rhododendron, Phytophthora ramorum
Introduction
Phytophthora ramorum, a pathogen of aerial plant parts, causes Ramorum shoot
dieback and leaf blight on many nursery species in Europe and North America and
sudden oak death (SOD) on certain members of the Fagaceae. It has been detected in
soil and potting media, but root infections have not been observed. The purpose of
these experiments was to determine if potting media infested with P. ramorum could
serve as a potential source of inoculum for potted rhododendrons, and to determine if
P. ramorum is capable of infecting roots.
Materials and Methods
Rhododendron ‘Nova Zembla’ plants grown from rooted cuttings and native Pacific
rhododendron (R. macrophyllum) plants grown from seed were transplanted into
potting media artificially infested with P. ramorum.
Inoculum consisted of V-8-broth-vermiculite cultures of P. ramorum or chopped
infested rhododendron leaves inoculated with Oregon nursery isolate 03-74-N11A
(European genotype, A1 mating type). A zoospore dose-response experiment was
1
A version of this paper was presented at the Sudden Oak Death Second Science Symposium: The
State of Our Knowledge, January 18-21, 2005, Monterey, California
2
Dept. of Crop and Soil Science, Oregon State University, Corvallis, OR 97331; Corresponding author:
Jennifer.Parke@oregonstate.edu
291
GENERAL TECHNICAL REPORT PSW-GTR-196
also conducted with Rhododendron ‘Nova Zembla’ using two isolates: 03-74-N11A
and 4143 (North American genotype, A2 mating type) and on R. macrophyllum with
isolate 03-74-N11A. A suspension of zoospores (3 x 105, 3 x 104, or 3 x 103
zoospores mL-1) was pipetted onto the surface of potting media in a 5-cm-diameter
circle around the base of the stem. Non-inoculated controls received water only.
Disease incidence based on above-ground symptoms was recorded every few days
for 6 weeks. As plants wilted and died, samples of roots, belowground stems,
aboveground stems and leaves were plated onto PAR selective medium for recovery
of the pathogen. Roots were surface-sterilized in 10 percent bleach solution for 30
sec. before plating. Duplicate tissue samples were processed for multiplex PCR
analysis. There were five or six replicate plants per inoculation treatment for each
host, and each experiment was conducted twice. In all experiments, plants were
watered only from the bottom to prevent splash dispersal of inoculum onto stems and
foliage.
Results
Both infested amendment treatments, and inoculation of potting media with higher
doses of zoospores, resulted in plant mortality. Disease incidence was greater among
plants inoculated with 03-74-N11A as compared to plants inoculated with 4143, and
plant response reflected zoospore inoculum dose. Leaves wilted and collapsed within
3 to 6 weeks and upwardly-expanding necrotic lesions became apparent on the stems
within 4 to 6 weeks. P. ramorum was isolated at high frequency from fine roots, large
roots, belowground stems, and aboveground stems of plants grown in artificially
infested potting media. Occasionally it was also recovered from petioles of
inoculated plants. Detection of the pathogen was confirmed with PCR analysis.
Control plants grown in non-amended, non-inoculated potting media remained
healthy and did not yield P. ramorum.
Discussion
Potted rhododendrons grown in potting media amended with inoculum of P.
ramorum became infected and showed symptoms of stem necrosis, leaf wilting, and
death. P. ramorum was isolated from roots and stems of infected plants. These results
are similar to those reported for other rhododendron dieback-causing Phytophthora
species that also were able to cause root infections (Benson and Jones 1980). Disease
transmission from infested potting media to above-ground plant parts and recovery of
P. ramorum from asymptomatic root tissue demonstrate the need to monitor potential
292
Proceedings of the sudden oak death second science symposium: the state of our knowledge
belowground sources of inoculum of P. ramorum for preventing pathogen spread on
nursery stock.
References
Benson, D.M. and Jones, R.K. 1980. Etiology of rhododendron dieback caused by four
species of Phytophthora. Plant Dis. 64:687-691.
293